Method of synthesising and growing nanorods from a metal carbide on a substrate, substrates thus obtained and applications thereof

ABSTRACT

The invention relates to a process for synthesizing nanorods of a carbide of one metal M1 on a substrate, which comprises: a) the deposition, on the substrate, of a layer of nanocrystals of oxide of the metal M1 and nanocrystals of oxide of at least one metal M2 different from metal M1, the M1 metal oxide nanocrystals being dispersed within this layer; b) the reduction of the M1 and M2 metal oxide nanocrystals into corresponding metal nanocrystals; and c) the selective growth of the M1 metal nanocrystals. The invention also relates to a process for growing nanorods of a carbide of one metal M1 on a substrate from nanocrystals of this metal, to the substrates thus obtained and to their applications: fabrication of Microsystems provided with chemical or biological functionalities, in particular the fabrication of biosensors; electron emission sources, for example for flat television or computer screens; etc.

TECHNICAL FIELD

The invention relates to a process for synthesizing metal carbidenanorods on a substrate, and more particularly chromium carbidenanorods, and to a process for growing such nanorods on a substrate fromnanocrystals of the same metal.

It furthermore relates to the substrates thus obtained and to theirapplications.

The synthesis and growth processes according to the invention result inthe formation of metal carbide nanorods which, apart from having a rigidand robust structure inherent in carbides, are firmly attached to thesubstrate on which their synthesis or their growth was carried out,perpendicular to the principal plane of this substrate, and arephysically separate from one another, i.e. are not in contact with oneanother.

These nanorods are therefore capable of being functionalized by thegrafting of organic, chemical or biological molecules and consequentlyare of most particular benefit for the fabrication of microsystemsprovided with chemical or biological functionalities, and moreparticularly the fabrication of useful biosensors, for example in thefields of medical research and analysis in clinical biology andagri-foodstuffs, especially for the control of manufacturing processesand quality control of raw materials and of end products, or else in theenvironmental field.

The nanorods are also capable of acting as field emission tips forelectron emission and thus of forming part of the construction ofelectron emission sources, for example for the fabrication of flattelevision or computer screens, or of being used to modify the opticalproperties of surfaces, such as for example luminescence with lowwavelength dispersion.

They may also find applications in the production of useful nanofluiddevices, for example in chromatography techniques.

PRIOR ART

In recent years, many processes for obtaining nanotubes, mainly carbonnanotubes, or nanorods have been proposed.

Table I, which is at the end of the present description, givesrepresentative examples of these processes, which are essentially ofthree types.

First, there are those which are aimed at producing nanotubes on powdersof catalyst crystals and which result in the formation of nanotubes thatare not attached to any substrate.

Thus, document [1] (Flahaut et al., J. Mater. Chem., 2000, 10, 249-252)describes a process for the preparation of carbon nanotubes, whichconsists in reducing an Mg_(0.9)Co_(0.1)O solid solution, by an H₂CH₄mixture in a furnace heated to 1000° C., in order to obtain a compositepowder formed from carbon nanotubes, from cobalt and from magnesiumoxide, and then in treating this powder with an acid in order to removethe cobalt catalyst.

Similarly, document [2] (Zhu et al., J. Mater. Chem., 2000, 10,2570-2577) discloses a process for the preparation of tungstendisulphide nanotubes, in which a powder composed of tungsten oxidenanorods or nanoneedles is reduced by hydrogen sulphide in a furnaceheated to 1100° C., and then the nanotubes thus formed are separatedfrom one another by subjecting the powder to ultrasound in an acetonebath.

Secondly, there are processes which aim to produce, on a substrate, anadherent film formed from nanotubes that are vertically erect buttouching one another.

Thus, document [3] (Bower et al., Appl. Phys. Lett., 2000, 77(6),830-832) relates to a process for obtaining a uniform film of carbonnanotubes on a silicon substrate, which process involves the depositionof carbon using MPECVD (Microwave Plasma Enhanced Chemical VapourDeposition), by decomposition of the acetylene present in a C₂H₂/NH₃mixture. The silicon substrate is precoated with a layer of cobalt about2 nm in thickness, this layer being intended to act as a catalytic seedfor growing the nanotubes.

The authors of document [4] (Zhang et al., Appl. Phys. A, 2002, 74419-422) have also obtained a uniform film of carbon nanotubes on aquartz substrate, but by CVD and using ethylene diamine as carbonprecursor. Here again, thermal deposition of a nickel film, capable ofacting as a catalytic seed, is produced beforehand on the substrate.

The third type of process groups together those employing a lithographyoperation for the purpose of obtaining, on a substrate, nano-objectsthat are both vertically erect and separated from one another.

Thus, document [5] (Hadobas et al., Nanotechnology, 2000, 11, 161-164)relates to a process for obtaining a grid pattern of siliconnantostructures on a substrate composed of this same material, whichprocess comprises the formation of a pattern by optical lithography bymeans of an argon laser, followed by oxygen plasma etching and thensulphur hexafluoride plasma etching. The nanostructures thus obtainedmeasure 35 to 190 nm in height, depending on the specimens, and areseparated from one another by 300 nm.

Document [6] (Ren et al., Appl. Phys. Lett., 1999, 75(8), 1086-1088)describes a process for the production of carbon nanotubes on a siliconsubstrate, which consists in producing, on this predoped substrate, anickel grid pattern by electron beam lithography followed by electronbeam evaporation, and then by depositing carbon by PEHFCVD (PlasmaEnhanced Hot Filament Chemical Vapour Deposition) using anacetylene/ammonia mixture, the nickel tips acting as seeds for thegrowth of the nanotubes.

Document [7] (Teo et al., Appl. Phys. Lett., 2001, 79(10), 1534-1536)presents a process that relies on the same principle, but in which thenickel pattern is produced by two successive lithographic steps, oneinvolving optical lithography and the other electron beam lithography,while the carbon deposition is obtained by DCCVD (Direct CurrentChemical Vapour Deposition) from an acetylene/ammonia mixture.

Document [8] (Fan et al., Physica E, 2000, 8, 179-183) proposes aprocess for obtaining bundles of carbon nanotubes on a silicon substraterendered porous beforehand, which process consists in depositing, onthis substrate, an iron film provided with uniformly spaced quadrangularapertures, by lithography followed by electron beam evaporation, andthen in causing the bundles of carbon nanotubes to grow by placing thesubstrate in a stream of ethylene in a furnace heated to 700° C.

Because the processes described in documents [5] to [8] include alithography operation, which is extremely expensive and can be carriedout only over limited areas, the use of these processes is inconceivablefor the production of nano-objects of the nanotube or nanorod type overlarge areas. In addition, the dispersion of the nanotubes on the surfaceof the substrate proves to be highly irregular in the case of document[6], whereas it is non-existent in the case of document [8], because thelatter case results in the formation of nanotubes joined together inbundles.

In document [9] (Chhowalla et al., J. Appl. Phys., 2001, 90(10),5308-5317) it has also been proposed, in order to grow verticallyaligned carbon nanotubes on a silicon substrate, to deposit, on thissubstrate, a thin film of a cobalt- or nickel-based catalyst bysputtering or thermal evaporation, then to sinter this catalyst film byheating to 750° C. and to deposit carbon by DCCVD using anacetylene/ammonia mixture.

Although this process has the advantage of not using lithography, itdoes not make it possible, however, to obtain a uniform distribution ofthe nanotubes and with a sufficient spacing between the latter.

Finally, document [10] (Li et al., Appl. Phys. Lett., 1999, 75(3),367-369) teaches a process for the growth of carbon nanotubes by thepyrolysis of acetylene on cobalt predeposited in the channels of analumina layer. However, the distance between these channels cannot beeasily controlled and, here again, the nanotubes obtained areinsufficiently far apart.

Thus, the only processes at the present time that allow the production,on a substrate, of vertically erect nano-objects separated from oneanother all include a lithography operation, which is both expensive andlimited to small areas.

The inventors were therefore set the objective of providing a processfor obtaining, on a substrate, metal carbide nanorods which are not onlyfirmly attached to this substrate, perpendicular to its principal plane,but are physically separate from one another, this being achievedwithout recourse to any lithography operation, so that this process canbe used for the production of nanorods over large areas and has a costpermitting it to be carried out on an industrial scale.

This and other objectives were achieved by the present invention, whichproposes both a process for synthesizing metal carbide nanorods on asubstrate and a process for growing such nanorods on a substrate fromnanocrystals of the same metal.

SUMMARY OF THE INVENTION

The subject of the invention is firstly a process for synthesizingnanorods of a carbide of one metal M1 on a substrate, which comprisesthe following steps:

a) the deposition, on this substrate, of a layer formed of nanocrystalsof oxide of the metal M1 and nanocrystals of oxide of at least one metalM2 different from M1, the M1 metal oxide nanocrystals being dispersedwithin this layer;

b) the reduction of the M1 and M2 metal oxide nanocrystals intocorresponding metal nanocrystals; and

c) the selective growth of the M1 metal nanocrystals.

According to the invention, step a) is preferably carried out byreactive sputtering from a target consisting of the metals M1 and M2 byan oxygen plasma produced by an ECR (electron cyclotron resonance)microwave plasma source.

The reactive sputtering from a metal target by a plasma of a gasproduced by an ECR microwave plasma source, as a technique fordepositing a metal or a metal oxide on a substrate, is now well known.The principle of this technique and an installation for achieving highmagnetic confinement allowing it to be implemented on substrates oflarge area, have been described by Delaunay and Touchais in Rev. Sci.Instrum., 1998, 69(6), 2320-2324 [11].

It will therefore simply be recalled that this technique consists ininjecting microwave power (for example at a frequency of 2.45 GHz) intoa plasma chamber consisting of one or more waveguides and including aregion of electron cyclotron resonance (for example at 875 gauss whenthe frequency of the microwave power is 2.45 GHz), thereby causingdissociation of the gas that is introduced into the plasma chamber andis at low pressure, generally below 10⁻³ mbar.

The ions and electrons thus created diffuse along the magnetic fieldlines and bombard a negatively biased metal target. The sputtering ofthis target in turn generates metal atoms that are deposited on thesubstrate located facing the target, thus forming a metal or metal oxidelayer on this substrate.

In the synthesis process according to the invention, sputtering of themetal target must result in the deposition, on the substrate, of a layerformed of nanocrystals of at least two different metal oxides.Specifically, this layer must comprise, on the one hand, nanocrystalsmade of oxide of the metal M1, that is to say the metal intended to formpart of the composition of the metal carbide nanorods that it is desiredto synthesize, and nanocrystals made of oxide of one or more metals M2different from M1, the role of which is to ensure that the M1 metaloxide nanocrystals are dispersed within this layer, so that the latterare physically separate from one another.

This is why the metal target used during step a) consists of both themetal M1 and the metal or metals M2 .

According to the invention, it is possible to adjust the flux of atomsof the metals M1 and M2 that are produced by the metal target when it isbeing sputtered and thus to control the density of the M1 metal oxidenanocrystals present in the layer of nanocrystals covering the substrateafter step a), by varying the composition of this target and/or itsbias.

Thus, in particular, the metal target may consist of a mixture of themetals M1 and M2, in which case it is subjected to one and the samenegative bias voltage over its entire area.

The metals M1 and M2 are therefore present in this mixture in atomicproportions (i.e. expressed as the number of atoms) which:

either corresponds to those in which they are desired to be found in thelayer of nanocrystals covering the substrate after step a), if it turnsout that the rate of sputtering of said metals M1 and M2 areapproximately the same under the chosen operating conditions;

or takes into account the differences that exist between the rates ofsputtering of the metals M1 and M2, if it turns out that these rates arenot the same under the chosen operating conditions.

As a variant, the metal target may comprise several zones, adjacent toone another or separated from one another, at least one of these zonesthen consisting of the metal M1, whereas the one or more other of thesezones consist(s) of the metal or metals M2 .

In this case, the flux of atoms of the metals M1 and M2 that areproduced by the various zones of the metal target may be obtained:

either by varying the respective areas of these zones, in which case itis possible to apply the same negative bias voltage to them;

or by varying the negative bias voltages that are respectively appliedto them, in which case the various zones may have the same area;

or else by varying both parameters, namely area and negative biasvoltage.

Whatever the situation, the choice of these parameters must take intoaccount any differences in sputtering rate that the metals M1 and M2have depending on the operating conditions.

The reduction of the M1 and M2 metal oxide nanocrystals deposited on thesubstrate during step a) into corresponding metal nanocrystals-or stepb) of the synthesis process according to the invention-is preferablycarried out by a hydrogen plasma produced by an ECR microwave plasmasource, the substrate then being heated.

Similarly, the selective growth of the M1 metal nanocrystals—or step c)of the synthesis process according to the invention-is preferablycarried out by a plasma of at least one hydrocarbon produced by an ECRmicrowave plasma source, the substrate also being heated.

It is thus possible to carry out all the steps of the synthesis processaccording to the invention by means of one and the same device, namelyan ECR microwave plasma source, this being a further advantage of theinvention.

This ECR microwave plasma source is preferably a source with highmagnetic confinement of the type described in document [11], allowingthe generation of low-pressure plasmas with highly energetic electronsand thereby ensuring a very high degree of dissociation of the gases inthe plasma chamber.

With regard to the foregoing, the metal M1 is preferably chosen frommetals capable of reacting, in step c), with organic molecules orradicals that are in gaseous form in order to form, with them, a metalcarbide and thus induce the growth of nanorods made of this carbide fromthe nanocrystals of this metal M1 .

Metals of this type are especially chromium and molybdenum, chromiumbeing preferred within the context of the invention.

The metal or metals M2 are chosen from metals having an affinity forcarbon-containing molecules or radicals that are in gaseous form, whichallows them, in step c) to fix these molecules or radicals bymetal-carbon bonds and makes it possible to form a protective graphiticlayer that blocks any growth from the nanocrystals of this or thesemetals M2 .

Such metals are those known as catalysts in organic chemistry. They arein particular iron, nickel and cobalt, iron and nickel being preferredwithin the context of the invention.

When the metal M1 is chromium, while the metal or metals M2 are chosenfrom iron and nickel, then step a) is preferably carried out by reactivesputtering of a target made of a stainless steel composed of iron andchromium, or of iron, chromium and nickel, such as for example anaustenitic stainless steel composed of 68% iron, 18% chromium and 14%nickel.

This target is advantageously biased with a negative voltage of −200 Vor higher and preferably of between −400 and −200 V, while the oxygenplasma is maintained at a pressure of generally 10⁻³ mbar or below, andpreferably between 10⁻⁴ and 10⁻³ mbar, so as to optimize the energy ofthe electrons produced by the plasma.

The other operating conditions, such as the frequency and the power ofthe electromagnetic wave delivered by the microwave generator, or thecurrents presented by the magnetic field at the point of injection ofthe microwave power and in the ECR zone, are similar to those generallyused in high-confinement ECR microwave plasma sources, and areespecially similar to those described by Delaunay and Touchais indocument [11].

Thus, after reactively sputtering the target for a time of about 20minutes, a layer of generally around 50 nm in thickness is obtained,which comprises chromium oxide nanocrystals disseminated between ironoxide nanocrystals and possibly nickel oxide nanocrystals, all thesenanocrystals typically having a diameter of approximately 100 to 500 nm.

Preferably, in step b) the hydrogen plasma is maintained at a pressureof 10⁻² mbar or below, and advantageously between 10⁻³ and 10⁻² mbar,while the substrate is heated to a temperature ranging from 300 to 600°C. depending on the rate at which it is desired to reduce the metaloxide nanocrystals.

Under these conditions, the chromium oxide, iron oxide and possiblynickel oxide nanocrystals are reduced to chromium, iron and, as the casemay be, nickel nanocrystals which typically measure around 5 to 100 nmin diameter, in the space of 5 to 20 minutes.

Moreover, in step c), it is preferred that the hydrocarbon plasma bemaintained at a pressure of 10⁻² mbar or below, and preferably between10⁻³ and 10⁻² mbar, and that the substrate be heated to a temperature of600° C. or higher, and preferably between 600 and 800° C., in order todeliver the activation energy needed for the growth of the carbidenanorods.

According to the invention, the hydrocarbon or hydrocarbons used in stepc) are chosen from alkanes, alkenes, and alkynes such as, for example,methane, ethane, propane, ethylene and acetylene, and mixtures thereof.

It is preferred to use ethylene.

Thus, a structure of the nail board type, formed of a substrate andchromium carbide rods of nanoscale diameter, i.e. typically around 5 to100 nm, is obtained, these rods being firmly attached to the surface ofthis substrate, perpendicular to the principal plane of the latter, andalso being physically separate from one another.

The length of these nanorods depends on the duration of step c) . Apriori, it is preferred within the context of the invention to producenanorods not exceeding 1 μm in length so that they retain a certainuprightness, according to the applications mentioned above to which theyare more particularly intended, but it is possible, however, to continuetheir growth for a long enough time to obtain a structure provided withrelatively entangled nanorods.

The substrate may be chosen from a wide variety of materials whosedeformation temperature is above the temperature at which the substratehas to be heated during step c), such as for example silicon, certainglasses such as borosilicates, quartz or even a metal or metal alloy,such as stainless steel. The substrate may also be solid or perforated,that is to say it maybe, for example, in the form of a grid.

In steps b) and c), this substrate may be heated between other ones bymeans of a substrate holder provided with heating means, such as forexample an electrical resistance heating element.

The subject of the invention is also a process for growing nanorods of acarbide of one metal M1 on a substrate, which consists in subjectingnanocrystals of the metal M1 dispersed within a layer of nanocrystals ofat least one metal M2 different from M1, said layer being depositedbeforehand on the substrate, to the action of a plasma of at least onehydrocarbon produced by an ECR microwave plasma source.

According to the invention, this growth process is preferably carriedout using the same metals M1 and M2 as those mentioned above, an ECRmicrowave plasma source with high magnetic confinement, of the typedescribed in document [11] and operating conditions similar to thoseused during step c) of the synthesis process described above. The ECRmicrowave plasma source may have a magnetic structure consisting ofeither coils (solenoids), as in document [11], or permanent magnets, asdescribed in FR-A-98/00777 [12].

The processes for the synthesis and growth of metal carbide nanorods ona substrate according to the invention have many advantages. To bespecific, apart from the advantages already mentioned, they also offerthat of allowing metal carbide nanorods to be produced on substrates oflarge area, that is to say in practice greater than several dm², and atcosts that are compatible with industrial operation.

The subject of the invention is also a substrate that has metal carbidenanorods attached to its surface, perpendicular to the principal planeof this substrate, and physically separate from one another.

Preferably, these metal carbide nanorods measure 5 to 100 nm in diameterand 100 nm to 1 μm in length.

Also preferably, these metal carbide nanorods are chromium carbidenanorods.

Owing to the remarkable properties that these nanorods have, in terms ofstrength, robustness, straightness and aspect ratio (length/diameterratio), the substrates that are provided therewith may be used in verymany applications.

In particular, they are suitable for forming part of the construction ofmicrosystems provided with chemical or biological functionalities, moreparticularly of biosensors, after the said nanorods have beenfunctionalized by the grafting of organic molecules such as, forexample, proteins, such as antibodies, antigens or enzymes, ornucleotide fragments (DNA or RNA) . Methods for carrying out suchgrafting are known per se.

The substrates according to the invention are also capable of formingpart of the construction of electron emission sources, for example forthe fabrication of flat television or computer screens, or of being usedto modify the optical properties of surfaces, such as for exampleluminescence with low wavelength dispersion.

They may also find applications in the production of useful nanofluiddevices, for example in chromatography techniques.

Apart from the above provisions, the invention also includes otherprovisions that will emerge from the rest of the description below,which refers to examples of implementation of the synthesis processaccording to the invention and examples of metal carbide nanorodsobtained by this process.

The rest of the description is given by way of illustrating, but notlimiting, the invention and with reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1, 2 and 3 are diagrams illustrating three examples of embodimentsof a metal target that can be used in step a) of the synthesis processaccording to the invention for depositing, on a substrate, a layercomprising 90% iron oxide nanocrystals and 10% chromium oxidenanocrystals, when this step a) is carried out by reactive sputtering ofsuch a target by an oxygen plasma produced by an ECR microwave plasmasource.

FIG. 4 is a diagram illustrating the reactions that occur during step a)of the synthesis process according to the invention, when this step iscarried out by reactive sputtering of a target made of an austeniticstainless steel by an oxygen plasma produced by an ECR microwave plasmasource.

FIG. 5 is a diagram illustrating the reactions that occur during step c)of the synthesis process according to the invention, when this step iscarried out by an ethylene plasma produced by an ECR microwave plasmasource.

FIG. 6 shows the mass spectrum of the dissociation of ethylene byelectron impacts, as obtained during step c) of the synthesis processaccording to the invention, when this step is carried out by an ethyleneplasma produced by an ECR microwave plasma source.

FIG. 7 is an SEM (scanning electron microscope) micrograph, at 30 000×magnification, showing the onset of chromium carbide nanorod growth on asilicon wafer, as observed during implementation of the synthesisprocess according to the invention.

FIG. 8 is an SEM micrograph, at 80 000× magnification, of chromiumcarbide nanorods synthesized on a silicon wafer by the synthesis processaccording to the invention.

FIG. 9 is an SEM micrograph, at 200 000× magnification, of a chromiumcarbide nanorod synthesized on a silicon wafer by the synthesis processaccording to the invention.

FIG. 10 is a TEM (transmission electron microscope) micrograph, at 300000× magnification, of chromium carbide nanorods synthesized on astainless steel grid by the synthesis process according to theinvention.

FIG. 11 shows the spectra obtained by energy loss spectroscopy (spectraS1, S2, S3 and S4) and the TEM images (images I1, I2, I3 and I4) for theiron, carbon, chromium and oxygen atoms present in chromium carbidenanorods synthesized by the synthesis process according to theinvention, the spectrum S1 and the image I1 corresponding to iron, thespectrum S2 and the image I2 corresponding to carbon, the spectrum s3and the image

I3 corresponding to chromium and the spectrum S4 and the image I4corresponding to oxygen.

In FIGS. 1 to 5, the same reference numbers serve to denote the sameelements.

EXAMPLES

Referring firstly to FIGS. 1, 2 and 3, these show schematically threeexamples of embodiments of a metal target that can be used in step a) ofthe synthesis process according to the invention for depositing, on asubstrate 11, a layer containing about 90% iron oxide nanocrystals andabout 10% chromium oxide nanocrystals, when this step a) is carried outby reactive sputtering of a metal target by an oxygen plasma produced byan ECR microwave plasma source.

These examples are intended to illustrate the possibility afforded bythe synthesis process according to the invention for adjusting thefluxes of iron and chromium atoms produced by the target during itssputtering and thereby the density of the chromium oxide nanocrystals inthe layer of nanocrystals covering the substrate after step a), byvarying the composition of this target and/or its bias.

The metal target shown in FIG. 1 is in the form of a plate 10 that isplaced facing the substrate 11, approximately parallel to the latter.This plate is connected to a voltage generator 12 for applying to it oneand the same negative bias voltage, for example −400 V, over its entirearea.

Since the iron and chromium sputtering rates are substantially identicalunder the same operating conditions, the target 10 consists of a mixtureof iron and chromium, for example a stainless steel, in respectiveatomic proportions of 90% and 10%.

The metal target shown in FIG. 2 is in the form of three plates, 10 a,10 b and 10 c respectively, which lie in the same plane facing thesubstrate 11, but which are slightly separated from one another. Theseplates are connected to a voltage generator 12 for applying the samenegative bias voltage, for example −100 V, to them.

The plates 10 a and 10 c are made of iron, whereas the plate 10 b ismade of chromium.

In order for their sputtering to result in the deposition, on thesubstrate, of a layer containing about 90% iron oxide nanocrystals andabout 10% chromium oxide nanocrystals, the sum of the areas of theplates 10 a and 10 c is approximately equal to nine times that of theplate 10 b.

The metal target shown in FIG. 3 is also in the form of three plates, 10a, 10 b and 10 c respectively, lying in the same plane facing thesubstrate 11 and slightly separated from one another. As previously, theplates 10 a and 10 c are made of iron, whereas the plate 10 b is made ofchromium.

However, this metal target is distinguished from that illustrated inFIG. 2 by the fact that, on the one hand, the sum of the areas of theplates 10 a and 10 c is equal to the area of the plate 10 b and, on theother hand, the plates 10 a and 10 c and the plate 10 b are connected totwo different voltage generators, 13 and 14 respectively.

This is because, in this case, the fluxes of iron and chromium atomsproduced by the target are adjusted by applying a higher negative biasvoltage to the plates 10 a and 10 c than that applied to the plate 10 b,for example −1000 V as opposed to −100 V.

Referring now to FIG. 4, this shows schematically the reactions thattake place during step a) of the synthesis process according to theinvention, when this step is carried out by reactive sputtering of atarget 10 made of an austenitic stainless steel, for example composed of68% iron, 18% chromium and 14% nickel, by an oxygen plasma produced byan ECR microwave plasma source with high magnetic confinement, of thetype described in document [11].

Of the components of this source, only the stainless steel target 10,the electrical voltage generator 12 to which it is connected, thesubstrate 11 on which it is desired to deposit the iron, chromium andnickel oxide nanocrystals, the two external field lines, 16 a and 16 brespectively, of the magnetic field and the four coils, 20 a, 20 b, 20 cand 20 d respectively, that generate this field have been intentionallyshown in FIG. 4.

As may be seen in FIG. 4, the oxygen present in the plasma chamber andat a low pressure, for example a few 10⁻⁴ mbar, dissociates owing to theeffect of the microwave power injected into this chamber, generatingelectrons (e⁻) and ions (O₂ ⁺, O⁺) that sputter the target 10.

This sputtering in turn generates fluxes of iron, chromium and nickelatoms that are deposited on the substrate 11 together with oxygen(O^(·)) atoms, giving rise to the formation of a layer 21 formed fromiron oxide (Fe₂O₃), nickel oxide (NiO) and chromium oxide (Cr₂O₃)nanocrystals, within which layer the chromium oxide nanocrystals (shownsymbolically by solid black circles in FIG. 4) are dispersed.

FIG. 5 is a schematic representation similar to that of FIG. 4, butwhich shows the reactions that take place during step c) of thesynthesis process according to the invention, when this step is carriedout by an ethylene plasma produced by an ECR microwave plasma source,with high magnetic confinement.

Shown in FIG. 5 are two elements that are absent from FIG. 4, as theyare unnecessary during step a), namely a removable shield 22 for thetarget 10 and a substrate holder 23 provided with heating means, forexample an electrical resistance heating element.

In what follows, the layer 21 of nanocrystals covering the substrate 11is considered to be formed from iron, chromium and nickel nanocrystalsand results from the reduction of a layer of iron, chromium and nickeloxide nanocrystals that is obtained as illustrated in FIG. 4.

The ethylene present in the plasma chamber and which is at a lowpressure, for example a few 10⁻³ mbar, dissociates owing to the effectof the microwave power injected into this chamber, generating electrons(e⁻) and reactive carbon species (C_(x)H_(y) ⁺ and C_(x)H_(y) ^(·)wherex=1 or 2 and y=0 to 4).

These reactive species, on the one hand, react with the chromium presentin the chromium nanocrystals on the surface of the substrate 11 in orderto form with it chromium carbide thus leading to the growth, from thesenanocrystals, of chromium carbide nanorods (shown symbolically by solidblack rectangles in FIG. 5) and, on the other hand, are fixed by theiron and nickel nanocrystals, which causes the formation of a protectivegraphitic layer that prevents any growth from the iron and nickelnanocrystals.

The examples that follow are intended to illustrate modes ofimplementing the process according to the invention.

Example 1 Synthesis of Chromium Carbide Nanorods on Silicon

Chromium carbide nanorods were synthesized on silicon substrates using,for the three steps a), b) and c), an ECR microwave plasma source withhigh magnetic confinement, similar to that described in document [11].

The operating conditions were the following:

-   -   step a): sputtering of a metal target by an oxygen plasma:        -   target used: austenitic stainless steel composed of 68% Fe,            18% Cr and 14% Ni        -   target bias: −400 V        -   oxygen pressure: 2×10⁻⁴ mbar        -   sputtering time: 20 minutes        -   thickness of the nanocrystal layer thus deposited:≈50 nm;    -   step b): reduction by a hydrogen plasma:        -   hydrogen pressure: 1.5×10^(×3) mbar        -   substrate temperature: 500° C.        -   reduction time: 10-20 minutes;    -   step c): growth by an ethylene plasma:        -   microwave power: 50-150 watts for a frequency of 2.45 GHz        -   ethylene pressure: 10⁻³−3 ×10⁻³ mbar        -   substrate temperature: 640° C.        -   growth time: 10-30 minutes.

FIG. 6 shows the mass spectrum for the dissociation of ethylene C₂H₄ byelectron impacts, as obtained under these operating conditions. Thisspectrum shows that the ethylene is strongly dissociated into atoms andions, namely H⁺, H₂ ⁺, C⁺, C² ⁺, CH⁺, CH₂ ⁺, etc, these being fragmentsof this dissociation.

Moreover, FIGS. 7 to 9 are SEM micrographs, at 30 000×, 80 000× and 200000× magnifications respectively, which show, in the first micrograph,the onset of growth of the chromium carbide nanorods on the substrateand, in the case of the other two micrographs, chromium carbide nanorodsas obtained at the end of step c).

As may be seen in FIGS. 8 and 9, these nanorods (Ø≈37 nm, L≈190 nm forthe nanorods shown in FIG. 8; Ø≈50 nm, L≈250 nm for the nanorod shown inFIG. 9) are fixed to the substrate perpendicular to its principal plane,are straight and, in addition, are physically separate from one another,in the present case by a distance of about 800 nm (FIG. 8).

Example 2 Synthesis of Chromium Carbide Nanorods on a Stainless SteelGrid

Chromium carbide nanorods were synthesized on a substrate consisting ofa stainless steel grid, also using, for the three steps a), b) and c),an ECR microwave plasma source with high magnetic confinement, similarto that described in document [11].

The operating conditions were the following:

-   -   step a): sputtering of a metal target by an oxygen plasma:        -   target used: austenitic stainless steel composed of 68% Fe,            18% Cr and 14% Ni        -   target bias: −400 V        -   oxygen pressure: 2×10⁻⁴ mbar        -   sputtering time: 20 minutes        -   thickness of the nanocrystal layer thus deposited:≈50 nm;    -   step b): reduction by a hydrogen plasma:        -   hydrogen pressure: 3×10⁻³ mbar        -   substrate temperature: 550° C.        -   reduction time: 10 minutes;    -   step c): growth by an ethylene plasma:        -   microwave power: 50 watts for a frequency of 2.45 GHz        -   ethylene pressure: 3×10⁻³ mbar        -   substrate temperature: 620° C.        -   growth time: 16 minutes.

Thus, the chromium carbide nanorods visible in FIG. 10, whichcorresponds to a TEM micrograph at 300 000× magnification, wereobtained.

Here again, these nanorods, which measure about 10 nm in diameter and alittle more than about 100 nm in length, are fixed to the substrateperpendicular to its principal plane, are straight and are alsophysically separate from one another, in this case separated by adistance of slightly greater than 100 nm.

FIG. 11 shows the spectra obtained by energy loss spectroscopy (spectraS1, S2, S3 and S4) and the transmission electron microscope images(images I1, I2, I3 and I4) for the iron, carbon, chromium and oxygenatoms present in these nanorods, the spectrum S1 and the image I1corresponding to iron, the spectrum S2 and the image I2 corresponding tocarbon, the spectrum S3 and the image I3 corresponding to chromium andthe spectrum S4 and the image I4 corresponding to oxygen.

These spectra and these images confirm that the nanorods synthesizedaccording to the invention do indeed mainly consist of chromium carbide,iron and oxygen both being present only in residual form. TABLE 1 Ref.Material Synthesis process Problems [1] C nanotubes Mg_(0.9)Co_(0.1)Osolid solution reduction by H₂/CH₄: no attachment to a φ ≈ 0.5-5 nm θ =1000° C. substrate high temperature [2] WS₂ nanotubes WO_(x) nanorods ornanoneedle reduction by H₂S: no attachment to a φ ≈ 30 nm θ = 1100° C.substrate high temperature [3] C nanotubes on Si MPECVD on Co layer: f =2.45 GHz; P = 5 kW; touching tubes φ ≈ 30 nm-12 μm θ = 825° C.; p = 20torr; gas = C₂H₂/NH₃ [4] C nanotubes on quartz CVD on Ni film: gas =ethylene diamine/N₂ touching tubes φ ≈ 80‥150 nm [5] Si nanostructureson Si optical lithography by Ar⁺ laser (λ = 458 nm) lithography:cost/area O₂ plasma etching, then SF₆ plasma etching [6] C nanotubes onSi electron beam lithography + evaporation → Ni lithography: cost/area φ≈ 150 nm grid irregular dispersion PEHFCVD: θ = 660° C.; p = 1; 10 torr;gas = C₂H₂/NH₃ [7] C nanotubes on Si optical lithography + electron beamlithography: φ ≈ 100 nm lithography → Ni pads cost/area DCCVD: 2 mm Cuanode; −600 V; θ =700° C.; difficult p = 10² torr; gas = C₂H₂/NH₃implementation [8] Bundles of C nanotubes electron beam lithography +evaporation → lithography: on Si perforated Fe film cost/area CVD: θ =700° C.; gas = C₂H₄ nanotubes joined into bundles [9] C nanotubes on Sicathodic sputtering or thermal insufficient evaporation → Ni or Co filmdistance between sintering: θ = 750° C. nanotubes DCCVD: 2 mm Cu anode:−600 V; θ = 700° C.; difficult gas = C₂H₂/NH₃ implementation [10] Cnanotubes on multi- Co electrochemical deposition insufficient distancehole alumina C₂H₂ pyrolysis: θ = 650° C. between nanotubes

BIBLIOGRAPHY

[1] E. Flahaut, A. Peigney, Ch. Laurent and A. Rousset, J. Mater. Chem.,2000, 10, 249-252.

[2] Y. Q. Zhu, W. K. Hsu, H. Terrones, N. Grobert, B. H. Chang, M.Terrones, B. Q. Wei, H. W. Kroto, D. R. M. Walton, C. B. Boothroyd, I.Kinloch, G. Z. Chen, A. H. Windle and D. J. Fray, J. Mater. Chem., 2000,10, 2570-2577.

[3] C. Bower, W. Zhu, S. Jin and O. Zhou, Appl. Phys. Lett., 2000,77(6), 830-832.

[4] W. D. Zhang, Y. Wen, W. C. Tjiu, G. Q. Xu and L. M. Gan, Appl. Phys.A, 2002, 74, 419-422.

[5] K. Hadobas, S. Kirsch, A. Carl, M. Acet and E. F. Wassermann,Nanotechnology, 2000, 11, 161-164.

[6] Z. F. Ren, Z. P. Huang, D. Z. Wang, J. G. Wen, J. W. Xu, J. H. Wang,L. E. Calvet, J. Chen, J. F. Klemic and M. A. Reed, Appl. Phys. Lett.,1999, 75(8), 1086-1088.

[7] K. B. K. Teo, M. Chhowalla, G. A. J. Amaratunga, W. I. Milne, D. G.Hasko, G. Pirio, P. Legagneux, F. Wyczisk and D. Pribat, Appl. Phys.Lett., 2001, 79(10), 1534-1536.

[8] S. Fan, W. Liang, H. Dang, N. Franklin, T. Tombler, M. Chapline andH. Dai, Physica E, 2000, 8, 179-183.

[9] M. Chhowalla, K. B. K. Teo, C. Ducati, N. L. Rupesinghe, G. A. J.Amaratunga, A. C. Ferrari, D. Roy, J. Robertson and W. I. Milne, J.Appl. Phys., 2001, 90(10), 5308-5317.

[10] J. Li, C. Papadopoulos, J. M. Xu and M. Moskovits, Appl. Phys.Lett., 1999, 75(3), 367-369.

[11] M. Delaunay and E. Touchais, Rev. Sci. Instrum., 1998, 69(6),2320-2324.

[12] FR-A-98/00777.

1-30. (canceled)
 31. A process for synthesizing nanorods of a carbide ofone metal M1 on a substrate, comprising the following steps: a) thedeposition, on the substrate, of a layer comprising nanocrystals ofoxide of the metal M1 and nanocrystals of oxide of at least one metal M2different from the metal M1, the M1 metal oxide nanocrystals beingdispersed within this layer; b) the reduction of the M1 and M2 metaloxide nanocrystals into corresponding metal nanocrystals; and c) theselective growth of the M1 metal nanocrystals.
 32. The process accordingto claim 31, wherein step a) is carried out by reactive sputtering froma target consisting of the metals M1 and M2 by an oxygen plasma producedby an electron cyclotron resonance microwave plasma source.
 33. Theprocess according to claim 32, wherein said target comprises a mixtureof the metals M1 and M2 .
 34. The process according to claim 32, whereinsaid target comprises several zones, adjacent to one another orseparated from one another, at least one of these zones consisting ofthe metal M1, whereas the one or more other of these zones consist(s) ofthe metal or metals M2 .
 35. The process according to claim 31, whereinstep b) is carried out by a hydrogen plasma produced by an electroncyclotron resonance microwave plasma source, the substrate being heated.36. The process according to claim 31, wherein step c) is carried out bya plasma of at least one hydrocarbon produced by an electron cyclotronresonance microwave plasma source, the substrate being heated.
 37. Theprocess according to claim 31, wherein the metal M1 is selected from thegroup consisting of metals capable of reacting with organic molecules orradicals that are in gaseous form in order to form, with them, a metalcarbide.
 38. The process according to claim 37, wherein the metal M1 isat least one selected from the group consisting of chromium andmolybdenum.
 39. The process according to claim 31, wherein the metal ormetals M2 are selected from the group consisting of metals utilized ascatalysts in organic chemistry.
 40. The process according to claim 39,wherein the metal or metals M2 are at least one selected from the groupconsisting of iron, nickel and cobalt.
 41. The process according toclaim 32, wherein said target comprises a stainless steel composed ofiron and chromium, or of iron, chromium and nickel.
 42. The processaccording to claim 40, wherein said target is biased with a negativevoltage of −200 V or higher and preferably of between −400 and −200 V.43. The process according to claim 41, wherein said oxygen plasma ismaintained at a pressure of generally 10⁻³ mbar or below, and preferablybetween 10⁻⁴ and 10⁻³ mbar.
 44. The process according to claim 35,wherein, in step b), the hydrogen plasma is maintained at a pressure of10⁻² mbar or below, and advantageously between 10⁻³ and 10⁻² mbar, andthe substrate is heated to a temperature ranging from 300 to 600° C. 45.The process according to claim 36, wherein, in step c), the hydrocarbonplasma is maintained at a pressure of 10⁻² mbar or below, and preferablybetween 10⁻³ and 10⁻² mbar, while the substrate is heated to atemperature of 600° C. or higher, and preferably between 600 and 800° C.46. The process according to claim 36, wherein the hydrocarbon orhydrocarbons used in step c) are at least one selected from the groupconsisting of alkanes, alkenes, alkynes, and ethylene.
 47. The processaccording to claim 31, wherein the substrate is at least one selectedfrom the group consisting of silicon, borosilicate glasses, quartz,metals and metal alloys.
 48. A process for growing nanorods of a carbideof one metal M1 on a substrate, which consists in subjectingnanocrystals of the metal M1 dispersed within a layer of nanocrystals ofat least one metal M2 different from M1, said layer being depositedbeforehand on the substrate, to the action of a plasma of at least onehydrocarbon produced by an ECR microwave plasma source.
 49. The processaccording to claim 48, wherein the metal M1 is selected from the groupconsisting of metals capable of reacting with organic molecules orradicals that are in gaseous form in order to form, with them, a metalcarbide.
 50. The process according to claim 49, wherein the metal M1 isat least one selected from the group consisting of chromium andmolybdenum.
 51. The process according to claim 48, wherein the metal ormetals M2 are selected from the group consisting of metals utilized ascatalysts in organic chemistry.
 52. The process according to claim 51,wherein the metal or metals M2 are at least one selected from the groupconsisting of iron, nickel and cobalt.
 53. The process according toclaim 48, wherein the hydrocarbon plasma is maintained at a pressure of10⁻² mbar or below and preferably of between 10⁻³ and 10⁻² mbar, whilethe substrate is heated to a temperature of 600° C. or higher, andpreferably of between 600 and 800°.
 54. The process according to claim48, wherein the hydrocarbon or hydrocarbons are at least one selectedfrom the group consisting of alkanes, alkenes, alkynes, and ethylene.55. The process according to claim 48, wherein the substrate is at leastone selected from the group consisting of silicon, borosilicate glasses,quartz, metals and metal alloys.
 56. A substrate comprising metalcarbide nanorods attached to its surface, perpendicular to the principalplane of this substrate, and physically separate from one another. 57.The substrate according to claim 56, wherein the metal carbide nanorodsmeasure 5 to 100 nm in diameter and 100 nm to 1 μm in length.
 58. Thesubstrate according to claim 56, wherein the metal carbide nanorods arechromium carbide nanorods.
 59. A method for the fabrication ofmicrosystems provided with chemical or biological functionalitiescomprising utilizing the substrate according to claim
 56. 60. A methodfor the fabrication of an electron emission source, a flat television ora computer screen comprising utilizing the substrate according to claim56.
 61. A method for the fabrication of biosensors comprising utilizingthe substrate according to claim 56.